1. Field of the Invention
This invention relates to a method for forming a crystal to be used for electronic materials, etc. according to the gas phase method.
2. Related Background Art
In the crystal formation methods in recent years, the Atomic Layer Epitaxy method having the characteristics of good controllability of crystal growth capable of effecting film growth one atomic layer and also suppressing the growth temperature at a lower level has been proposed (Usui, Sunakawa, Jap. J. Appl. Phys. 25 (1986), 212).
This is a technique in which a thin film is grown by feeding alternately reactive starting gases without mixing and allowing them to react on the surface of a substrate.
However, the above Atomic Layer Epitaxy method must use a monocrystal substrate, and therefore the cost is high, and there is also involved the problem that enlargement of area can be done with difficulty.
On the other hand, the experimental results about selectivity for crystal formation on any desired substrate are described in Classen, J. Electrochem. Soc. Vol. 127, No. 1, January 1980, 194-202.
The contents described in the above literature include preparation of two kinds of substrate having the surface comprising a material (Si.sub.3 N.sub.4) on which formation of a film to be deposited can be easily done and the surface comprising a material (SiO.sub.2) on which formation of a film to be deposited can be difficultly done and confirmation on the trial basis of selectivity of formation of silicon crystal on those substrates.
The above literature suggests that a crystalline deposited film can be formed selectively on a desired substrate by utilizing selectivity for crystal formation as described in the above literature. However, even so, there is nothing suggested about the method for forming a monocrystal deposited film at a desired position on the surface of a desired substrate.
Also, from the contents of description in the above literature, nothing is suggested about formation of a polycrystal by controlling the grain sizes of crystals with good precision.
Accordingly, it has been earnestly desired to have a technique for forming a monocrystal with good controllability at a desired position on a substrate by using any desired substrate which can suppress the cost at a lower level.
As the technique earnestly desired as mentioned above, European Published Patent Application No. 0,244,081 filed by the assignee of the present application discloses formation at any desired position on the substrate by utilizing the selective nucleation method at a cost suppressed at a lower level and formation of a polycrystal by controlling the crystal grain sizes of monocrystals with good precision.
To show the crystal formation method utilizing the selective nucleation method disclosed in the above European Published Patent Application No. 02440821 as an example for better understanding of the present invention, the selective nucleation method, to show it conceptionally by way of example, is a method in which on a substrate having a non-nucleation surface (S.sub.NDS) having small nucleation density and a nucleation surface (S.sub.NDL) arranged adjacent to the non-nucleation surface (S.sub.NDS), having sufficiently small surface area for permitting crystal growth only from a single nucleus and larger nucleation density (ND.sub.L) than the nucleation density (ND.sub.S) of the non-nucleation surface (S.sub.NDS), crystal growth is effected originating from the single nucleus formed on the above nucleation surface (S.sub.NDL), and further the monocrystal is grown beyond the nucleation surface (S.sub.NDL) even to cover over the non-nucleation surface (S.sub.NDS).
The above method for forming a crystal has various excellent industrial advantages, but for utilization for various uses, it is desirable to further improve completeness of monocrystal quality, yield, productivity, bulk productivity, etc.
The present inventor, in view of the above points, aims at improving further the above method for forming a crystal so that it can be optimized for preparation of a crystal utilizable for formation of various electronic devices, optical devices, etc. of high performance and high characteristics.
As the results of further studies made by the present inventor according to the selective nucleation method, the present inventor has found that in the selective nucleation method, when the crystal growth treatment is performed by the gas phase method by use of reactive starting gases, the starting gases will react with each other in the gas phase, whereby two points antagonistic to each other exist, and that the measure for solving the problems obtained from these knowledges can give better embodiments of the present invention. These solving measures can apply the selective nucleation method for broader scope of uses.